1. Field of the Invention
This invention relates to a thermally-assisted recording (TAR) type of magnetic recording disk drive that uses patterned media, wherein each data bit is stored in a magnetically isolated island on the disk, and more particularly to servo control and write synchronization in this type of disk drive.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media, also called bit-patterned media (BPM), have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated data islands or islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the regions between the islands must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is substantially no magnetic material in the regions between the islands.
In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define troughs or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with relatively low-cost, high volume processes such as lithography and nanoimprinting. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. The trenches are recessed far enough from the read/write head to not adversely affect reading or writing.
Patterned-media disk drives, like conventional continuous-media disk drives, are also required to have a servo control system for positioning the read/write heads to the desired tracks and maintaining them on track during reading and writing. This is typically accomplished with dedicated nondata servo regions or sectors angularly spaced around the disk. In patterned-media disks, nondata servo regions have been proposed that contain discrete servo islands or blocks separated by nonmagnetic spaces. The servo blocks are patterned into a position error signal (PES) field that generates a servo readback signal that is detected by the read head and demodulated into a PES for positioning the read/write head to the desired data track and maintaining it on track.
In addition to nondata servo regions, patterned-media disks have been proposed that also have nondata synchronization fields for write synchronization. Unlike conventional magnetic recording where the data bits are written on continuous media, the magnetic transitions between discrete data islands in patterned-media disks are restricted to predetermined locations governed by the locations of individual data islands. It is thus necessary to synchronize the reversal of write current in the write head with the passing of individual data islands under the head. The nondata synchronization fields are detected by the read head and used to synchronize the write head. U.S. Pat. No. 7,675,703 B2, assigned to the same assignee as this application, describes a patterned-media disk drive with nondata write synchronization fields.
Heat-assisted magnetic recording (HAMR), also called thermally-assisted recording (TAR), has been proposed. In TAR systems, an optical waveguide with a near-field transducer (NFT) directs heat from a radiation source, such as a laser, to heat localized regions of the magnetic recording layer on the disk. The radiation heats the magnetic material locally to near or above its Curie temperature to lower the coercivity enough for writing to occur by the write head. TAR systems have been proposed for patterned-media disk drives, wherein each data island is heated so that the coercivity of the magnetic material is lowered when the magnetic write field from the write head is applied to the island.
Patterned-media TAR disk drives also require servo control systems and write synchronization. The servo control systems proposed for patterned-media TAR disk drives are the same as for patterned-media disk drives without thermal assistance and thus use magnetic nondata servo regions angularly spaced around disk that are detected by the read head. Magnetic servo schemes suffer from the problem that the write clock is only updated every servo sector, thus making the system susceptible to disturbances and drift between the updates. Packing the servo sectors closer together uses up storage space. Schemes that do not rely on magnetic readback of the nondata servo regions have the benefit of not needing to stop the write process during servo readback because the servo signal is not corrupted by the magnetic fields generated during the write process. This allows a lower overall storage overhead for implementing the servo.
For write synchronization, an optical technique has been proposed that uses the actual data islands rather than dedicated synchronization fields. For example, U.S. patent application Ser. No. 12/209,089, filed Sep. 11, 2008, published as US 2010/0061018 A1 and assigned to the same assignee as this application, describes a patterned-media TAR disk drive that uses a radiation detector instead of the read head to detect reflected radiation from the data islands, with the radiation detector output being used to control the clocking of write pulses from the write head. However, the radiation absorption contrast can be quite low in systems that seek to maximize areal density and the thermal stability and writability of the data islands. Radiation variation detection schemes work best for data islands that have wide gaps between island edges, which is not desirable for high density BPM.
What is needed is a patterned-media TAR disk drive that has servo control and write synchronization that does not rely on magnetic readback from nondata regions, but which still provides adequate signal-to-noise ratios.